It is common practice to isopress (i.e., exert even pressure from all directions on) formed powders, powder compacts and composite intermediates prior to sintering (i.e., agglomeration by heating) to make high density ceramic, metal, and glass parts. Isopressing imparts a high uniform density throughout the part, which in turn assists the sintering process by allowing for a high and uniform sintered density at a lower sintering temperature. The result is a part having an improved microstructure, higher strength, and greater wear resistance.
There are several known isopress techniques. Two generally known isopress techniques include the "dry bag" and "wet bag" techniques. In the "dry bag" technique, a formed powder or porous powder compact is placed in the interior of an elastomeric bag or tube within the isostatic press. The elastomeric bag or tube is surrounded by a fluid which can be pressurized in order to compress the article placed therein. After compaction, the pressure is released and the treated part may be removed as a compressed intermediate for further processing.
In the "wet bag", the powder or pressed powder part is placed in an elastomer bag, sealed and placed in the isopress chamber. The chamber is then pressurized by pumping a liquid into the chamber. The pressure is uniformly applied to the bag, and the material within the bag, causing it to pack uniformly. Commonly used pressures range from about 34 MPa to 414 MPa.
A problem with the "wet bag" technique is that it is impractical to isopress small parts or parts with complex shapes. Small parts require a large number of elastomer bags in order to have productivity. Each of these bags would have to be individually handled. Complex parts require bags having complex shapes, which increases the expense and makes it difficult to insure that a complex shape will compact uniformly without cracking.
Another approach known as hot isostatic pressing (also known by the acronym "HIPing") incorporates forming and thermal treatment in one step. That approach has been used to obtain a dense ceramic from starting material that cannot be fully densified using conventional firing methods. However, it is usually limited to very simple shapes of a variety of materials and slow rates of production, and it is generally expensive.
The present invention solves these difficulties. In the present invention, a flexible coating is formed directly on the parts to be isopressed, permitting small parts or parts with complex shapes to be coated individually or in bulk. Higher production rates, reduced production cost, and the ability to produce complex and dense shapes from a variety of materials which could not previously be isopressed on a commercial scale can now be achieved with the present invention.